Mucopolysaccharidosis type IIIA (MPS-IIIA) is an inherited disease caused by the deficiency of sulfamidase (SGSH), an enzyme involved in the stepwise degradation of large macromolecules called heparan sulfates. As a consequence, undegraded substrates accumulate in the cells and tissues of the affected patients causing cell damage. The central nervous system (CNS) is the predominant target of damage and in fact, MPS-IIIA patients show severe mental retardation and neuropathological decline that ultimately leads to death (often <20 years). Clinical symptoms include hyperactivity, aggressive behaviour and sleep disturbance (1).
A naturally occurring mouse model of MPS-IIIA has been identified with pathophysiology and symptoms that resemble the human condition (2-4). These mice represent an ideal model to study the physiopathology of this disorder and to test new therapeutic protocols.
The treatment of brain lesions represents the principal goal of any therapeutic approach for MPS-IIIA. A route to reach the brain consists in the direct injection of a therapeutic molecule directly into the brain. A number of different enzyme replacement therapy (ERT) protocols have been tested. In these protocols, a recombinant sulfamidase enzyme was administered through the direct injection into the brain of MPSIIIA mice. These strategies are able to delay the appearance of neurodegenerative changes when sulfamidase is administered in the younger mice (5, 6). In addition, a Gene Therapy protocol based on the intracerebral injection of the SGSH gene via AAV vectors was successfully developed by the authors of the invention (7). Although these direct brain-targeting approaches have been shown to be clinically effective they represent highly invasive approaches for human therapeutic applicability.
Since every neuron in the brain is perfused by its own blood vessel, an effective alternative low-invasive route to reach the brain is the intravenous administration of the therapeutic molecule (8). However, this very dense network of microvasculature, which forms the Blood-Brain Barrier (BBB), is not permeable to all the molecules and might impede effective delivery of therapeutic agents (9). Indeed, intravenous administration of lysosomal enzymes has produced a therapeutic effect on the somatic pathology of many LSDs but it has no or little effect on the CNS pathology due to the impermeability of the BBB to large molecules (10). In MPS-IIIA, it has been demonstrated that intravenous injection of sulfamidase does not alter the pathology or behavioural process occurring in the MPS-IIIA mouse brain when the enzyme is supplied after the BBB has been formed (11).
Importantly, a recent study by Urayama et al. demonstrated that sulfamidase is transported across the BBB in neonatal mice throughout the mannose 6-phosphate receptor-mediated transport but the influx into adult brain was negligible (12).
It is clear that in such context the real challenge for the therapy of MPS-IIIA and in general for all LSDs involving the CNS is to develop CNS systemic treatment strategies that can overcome the major obstacle represented by BBB. An effective strategy to cross the BBB is the targeting of proteins to the CNS via receptor-mediated transcytosis (13). Well-characterized BBB receptors include: low density lipoprotein receptor (LDLR), the transferrin receptor (TfR), and the insulin-like growth factor receptor (IGF-R). The LDLR family represents a group of cell surface receptors that binds apolipoprotein (Apo) complexes (lipid carriers) for the internalizing into the lysosomes (14-16). On the surface of the BBB, LDLR binding to Apo results in the transcytosis to the luminal side of the BBB, where the apolipoprotein is released to be uptaken by neurons and astrocytes. A recent study has demonstrated that fusing the LDLR-binding domain of Apo to a lysosome enzyme results in an efficient delivery of the chimeric enzyme to the CNS (17).
WO2004108071 refers to a chimeric CNS targeting polypeptide comprising a BBB-receptor binding domain, such as the Apolipoprotein B binding domain, for therapeutic use in lysosomal storage diseases.
WO2004064750 refers to nucleic acids encoding a chimeric lysosomal polypeptide (specifically the lysosomal acid glucosidase GAA implicated in the lysosomal storage disorder Glycogen storage disease type II) comprising a secretory signal sequence (i.e. Vi-antitrypsin and alpha-1-antitrypsin) and the related AAV vectors.
WO2005002515 refers to a compound comprising a megalin-binding moiety conjugated to an agent of interest for receptor mediated drug delivery, particularly by transcytosis, across the blood-brain barrier. Moreover the document refers to a method of treating a lysosomal storage disease based on the administration of a composition comprising a megalin-binding moiety. Apolipoprotein B and Mucopolysaccharidosis IIIA are mentioned.
WO2009131698 refers to a therapy based on a chimeric NaGlu enzyme characterized by an Apolipoprotein B binding domain and directed specifically to Mucopolysaccharidosis IIIB.
Cardone et al. (Hum Mol Gen, 2006 15(7):1225) describes the correction of Hunter syndrome (the lysosomal storage disease Mucopolysaccharidosis Type II) in the MPSII mouse model by liver-directed AAV2/8-TBG-mediated gene delivery.
WO2007092563 refers to a method and compositions for tolerizing a mammal's brain to exogenously administered acid sphingomyelinase polypeptide by first delivering an effective amount of a transgene encoding the polypeptide to the mammal's hepatic tissue and then administering an effective amount of the transgene to the mammal's central nervous system (CNS). The therapeutic approach is directed to Niemann-Pick disease, a lysosomal storage disease. Liver-specific promoters and AAV type 8 are mentioned.
WO2009075815 refers to methods of treating Pompe disease (a lysosomal storage disease) which involves the administration of an AAV vector in the context of enzyme replacement therapy. Liver-specific promoter (thyroid hormone-binding globulin promoter) and AAV type 8 are mentioned.
None of the above prior art cited documents disclose or even suggest the modified sulfamidase enzyme of the instant invention and that it may have a therapeutic effect for the treatment of MPS type IIIA.